The idea is to have the moving boundary zone never displace by more than 1 cell length. Hence as a rule of thumb, the time step should be less than the smallest cell length divided by largest speed of moving boundary.
You should also set the fluid cell zone, in which the moving boundary zone is moving into, to deforming with the appropriate remeshing settings.
You can find a tutorial on 6-DOF in our Fluent Help page Chapter 13: Using Overset and Dynamic Meshes (ansys.com). Obviously this is not the same case as you (ignore Overset Meshing section) but best practices are described therein, including initializing the flow with a steady-state simulation prior to setting dynamic mesh setup to facilitate convergence.
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